Radius Arm Bushes

A radius arm runs from the axle back to the chassis, stopping the axle walking forward under power and backward under braking. It also resists the axle housing rotating, which is what sets caster on a solid front axle.

Radius arm bushes are the only thing holding that arm in position, and they take every bit of load the vehicle generates through the front end.

What Radius Arm Bushes Do and Why They Set Caster

Because the radius arm controls how far the axle housing rotates, it also controls caster, which is the angle that makes your steering settle and return to centre after a corner.

When the bushes are healthy the housing stays where it belongs and caster holds steady. When they soften, the housing is free to rotate slightly under load, so caster changes every time you brake, accelerate or hit a bump.

What you feel is a vehicle that wanders and needs constant correction, steering that goes vague as speed rises, and in the worst cases a shimmy coming back through the wheel. None of it points obviously at a bush the size of your fist at the end of an arm nobody looks at.

That connection is the reason people search for radius arm bushes, caster bushes and radius rod bushes and all end up looking at the same component.

Why Radius Arm Bushes Have to Be Firm and Flexible at Once

Most suspension bushes manage load in one direction. Radius arm bushes have two requirements that pull directly against each other.

They have to be firm enough to hold the arm against braking and acceleration load, because any give there is caster moving around while you drive. At the same time they have to twist, because when one wheel climbs and the other drops the axle rotates relative to the chassis and the arm has to accommodate that movement.

Restrict it too far and the suspension binds, the wheel stops following the ground, and the arm starts fighting the axle instead of locating it. Factory rubber resolves that conflict by simply being soft, so it handles the articulation and gives up control of everything else, then gets worse at both jobs as it hardens with age.

Polyurethane radius arm bushings hold their shape under load while still allowing the arm to twist, which is what lets them do both jobs rather than compromising on one.

Caster Bushes, Castor Bushes and Radius Rod Bushes Are the Same Part

The terminology around this position is inconsistent, so it is worth clearing up before you order.

Depending on the vehicle and the catalogue, the same component is listed as radius arm bushes, caster arm bushes, caster rod bushes or radius rod bushes. You will also see caster spelled castor in plenty of Australian listings, and castor bushes means exactly the same thing.

On a strut front end the equivalent part is the caster rod or radius rod, and front radius rod bushes fail the same way. They take braking load directly, so when they go soft the wheel moves rearward under brakes and the front end squirms.

Go by the position on the vehicle rather than the name in the listing, since the same words describe different parts across different platforms.

Why 4WDs Go Through Radius Arm Bushes Faster

Radius arm bushes wear out on any vehicle that has them, and a working 4WD gets there considerably sooner than a passenger car.

The loads are the first reason for that, because a vehicle carrying a canopy, drawers, long range tanks and a trailer is putting more through the front end every time it stops, and corrugated dirt roads cycle the bushes constantly rather than occasionally. Articulation does the same thing from the other direction, because every time one wheel climbs and the other drops, the bush is being twisted through a bigger arc than it ever sees on sealed road.

Contamination is the second reason and it does more damage than load. Water crossings, dust and mud all reach the bush, and once grit gets between the bush and the pin it grinds rather than pivots. That is why a vehicle that tours will often need these replaced while a city car of the same age and kilometres still has serviceable bushes.

If your vehicle works for a living, treat these as a wear item with a service life rather than something that lasts until it breaks.

Symptoms of Worn Radius Arm Bushes

• Steering that wanders and needs constant correction, worse at higher speeds
• Caster shimmy or a shake coming back through the steering wheel
• Clunking or knocking from the front under braking and on take up of drive
• A front end that seems to move before the vehicle responds to your input
• Uneven tyre wear that keeps returning after every wheel alignment
• Bushing material cracked, split, squeezed out or missing at either end of the arm
• Bent washers at a mounting point, which mean the arm has moved in a joint that should have been clamped tight
• Elongated holes at the mount, which usually means more than a bush needs attention
• Reduced traction on rough ground as the axle moves around under the vehicle

The bent washer is worth knowing about because almost nobody checks for it. It is evidence the arm has taken a hard enough hit to move where it should not, and finding one changes what you order.

Why Worn Radius Arm Bushes Never Show on the Alignment Rack

This is the part that leaves people chasing the fault for years without resolving it.

A wheel alignment is measured with the vehicle stationary. The loads that make a worn radius arm bush deflect are braking, acceleration and articulation, and none of them exist while the vehicle is sitting on the machine. The printout reads correctly, the workshop says the geometry is fine, and the vehicle still wanders the moment you drive away.

If caster reads low on the printout, that is a different signal and usually means ride height has changed rather than the bushes being worn. If the numbers read correctly and the vehicle still will not track straight, the bushes are where to look.

Standard Radius Arm Bushes or Caster Correction Bushes

Both are available and they solve different problems.

Standard replacements restore the geometry the vehicle is meant to have, which is what you want when the bushes have simply worn out and the ride height has not changed.

Caster correction bushes have the bore moved off centre, so fitting them shifts the arm pivot and recovers caster the vehicle has lost. They work because the arm is a lever, which means a small movement at the pivot becomes a useful angle at the wheel. Front caster bushes in an offset design are the standard fix for a mildly raised 4WD that has washed off caster and has no adjustment left.

Because the offset is directional, orientation decides what the bush does. Fitted the wrong way round it moves caster in the opposite direction and leaves the alignment further out than when you started.

How Much Caster Correction Bushes Can Actually Recover

Worth knowing the ceiling before you order, because it decides whether a bush will do the job at all.

An offset bush will typically hand back a degree or two depending on the position and the design. That covers a modest lift, a vehicle whose geometry has drifted with age, and the fine tuning an alignment needs to come back into range.

Once a vehicle needs four degrees or more of caster, there is nowhere left for a bush to move the pivot. At that point the correction has to come from a radius arm built with the offset designed into its geometry, or from repositioning the mounting point altogether.

The simple test is what the aligner tells you. If the machine gets close and just runs short, a bush will finish the job. If it is a long way out, buy the arm.

What Radius Arm Bush Replacement Costs and Where the Money Goes

The bushes themselves are inexpensive, and the labour to reach them is what determines the bill.

The arm has to come off the vehicle whichever route you take, and most positions need a press to get the old bushes out and the new ones in square. That work takes the same time whether you replace one bush or every bush on the arm, which is why doing one end now and the other in a year costs considerably more than doing both at once.

A wheel alignment afterward is not optional either, since replacing these changes caster the moment the vehicle sits back down on its wheels.

When comparing quotes, the numbers that move the total are how many positions are being done, whether the arms have to come off, whether correction bushes or standard replacements are being quoted, and whether the alignment is included in the price.

Pressing Radius Arm Bushes In and Getting Orientation Right

Support the vehicle and take the load off the suspension before releasing either end of the arm.

Most positions need pressing, and a correct diameter press tool is what protects the arm eye. Check that eye once the old bush is out, since a bore worn oval will not seat a new one properly and means the arm itself needs replacing rather than rebushing.

Fit the crush tubes and sleeves supplied, because the tube carries clamping load from the bolt and a bush fitted without one gets crushed as soon as it is torqued.

Pay close attention to orientation on any offset bush. Mark the intended position before pressing and check it against the listing rather than trusting your memory halfway through the job.

Grease on assembly with a lubricant made for polyurethane. Silicone and PTFE based greases are correct, while ordinary lithium and petroleum based greases can degrade the material over time.

Torque with the vehicle on its wheels at ride height rather than at full droop. This matters more here than at most positions, because the arm sits under constant fore and aft load and a bush clamped in the wrong position binds from the first kilometre.

Do both sides, then get a wheel alignment. Replacing these changes caster immediately, and the alignment is what confirms both sides now match each other.

Find Radius Arm Bushes for Your Vehicle

If the arm itself is bent or worn out, replace it with one of our individual radius arms.

Select your vehicle to see the standard and caster correction radius arm bushes available for it, along with the front caster rod positions on strut front ends. Every Nolathane component is backed by our warranty, a fitment guarantee and 60 day returns.